Using animal models, we have shown that mechanical pulmonary ventilation as now practiced can under certain conditions become highly injurious, and lethal (adult respiratory distress syndrome = ARDS). This injury process is related to peak inspiratory pressures, and duration of mechanical pulmonary ventilation. Using such an animal model of ARDS we have managed sheep on extracorporeal veno-venous blood gas exchange, while discontinuing all mechanical pulmonary ventilation. Within 24 hours of change in treatment to extracorporeal blood gas exchange, there was rapid improvement in pulmonary function in all animals, and 9/11 sheep were weaned off bypass and to room air. In a control group of animals treated with conventional continuous positive pressure ventilation (CPP) only 3/11 sheep could be weaned off bypass. This animal model of ARDS can become sufficiently severe so that no form of presently available therapy will lead to survivors. Such understanding only emphasizes the as yet unrecognized dangers implied in the use of CPPV.